1. Field of the invention
The invention relates to a process for the preparation of optionally substituted benzyl-benzenes by reaction of optionally substituted benzyl alcohols or the dibenzyl ethers produced from them with optionally substituted benzenes in the presence of activated bleaching earth at elevated temperature.
Optionally substituted benzyl-benzenes are, for example, diphenylmethane, benzyl-toluene, dibenzyl-benzenes, dibenzyl-toluenes, benzyl-biphenyl etc. The compounds mentioned are known to the person skilled in the art.
2. Description of the related art
Benzyl-benzenes of the type mentioned can be prepared by known methods by reaction of suitable aromatics with formaldehyde. Now, recently, apart from the toxic properties of many aromatics used, the maximum allowable concentration (MAC) for formaldehyde has been very greatly reduced. This means high outlays on work safety and on the effective suppression of any emission into the air or the waste water. By means of this, formaldehyde chemistry of this type is made prohibitively expensive.
In other, likewise known processes, suitable aromatics can be benzylated using benzyl chloride, for example in the presence of Friedel-Crafts catalysts. However, both the catalysts used in this case and a residual chlorine content or chlorine eliminated from the benzyl chloride produce large corrosion problems for the apparatus used in such processes. The resultant benzylated products retain a residual chlorine content which makes them thermally unstable and therefore makes use, for example, as a heat transfer oil problematical.
In addition, benzal chloride is always formed at the same time in the preparation of benzyl chloride by side chain chlorination of toluene. However, benzyl chloride and benzal chloride can only be separated by distillation in expensive nickel columns on account of the corrosion problems.
In principle, it should be possible to replace the benzyl chloride by benzyl alcohol for the purposes of benzylation. Such a replacement could have substantial advantages: first of all, benzyl alcohol would not produce the feared chloride ion corrosion in such a process; chloride ion corrosion occurring in the preparation of benzyl alcohol from benzyl chloride would in any case remain in the preparation process from benzyl alcohol or benzyl chloride, where it must always be overcome if it is intended to prepare benzyl chloride or benzyl alcohol for other purposes than those of the present invention. Secondly, the separation of the benzaldehyde always additionally resulting (from inevitable benzal chloride) in the preparation of benzyl alcohol from benzyl chloride might be possible in a normal steel distillation column (instead of the above-mentioned nickel column). Therefore attempts have already been made to carry out the benzylation with benzyl alcohol in the presence of sulphuric acid or phosphoric acid. However, such attempts produced large amounts of resinous reaction products in addition to the new corrosion by the strong acids mentioned.
It has also already been attempted to replace the strong mineral acids mentioned by bleaching earth in the benzylation with benzyl alcohol. Attempts of this type have been published at long intervals, without hitherto becoming economically useful.
Thus, DE-PS 638,756 discloses the reaction of 2 parts of benzyl alcohol with 4 parts of benzene (molar ratio 1:2.77) at 230.degree. C. in an autoclave. 50% by weight of Tonsil, relative to the benzyl alcohol, are employed for this. Diphenylmethane is prepared in a yield of about 40% of the theoretical yield and dibenzyl-benzenes in a yield of about 25%, all yields relating to benzyl alcohol.
Benzylation of some aromatics with benzyl alcohol in the presence of askanite is known from J. General Chemistry of the USSR (English translation of Z. obsz. Zhim.) 20 (1950), 2249 (page 2168 in the original). This procedure is carried out at 90.degree. C. and at 10% by weight of askanite, relative to the benzyl alcohol. If 200 g of benzyl alcohol and 200 g of benzene (molar ratio 1:1.38) are used, about 16% of diphenylmethane and 4.6% of dibenzyl-benzene, all relative to benzyl alcohol, are obtained; the non-distillable residue is larger than the total amount of diphenylmethane and dibenzyl-benzene. In the benzylation of toluene under the same conditions 10.8% of dibenzyl-toluene, both relative to benzyl alcohol, are obtained.
Reworking of both literature sources gave, in addition to the low yields, dark brown reaction products which in this form can be put to no further use.